Mesopic vision, a combination of rod and cone vision in modest but not low lighting situations, is a fundamental but challenging research area. It is fundamental because the interactions between rod and cone signals alter several aspects of visual function, such as color vision and temporal vision. It is challenging because traditional methodology has failed to yield a clear understanding of its complex role in human vision. The proposed research will use a novel four-primary photostimulator that independently controls rod and cone excitations. The study will combine psychophysical and physiological methodologies to investigate the neural mechanisms that mediate mesopic vision. Psychophysical experiments will be conducted to assess rod-cone interactions in color vision and temporal vision at mesopic light levels. Physiological experiments will be conducted to measure rod and cone inputs to ganglion cells in primates under mesopic illuminations. The goal is to determine retinal pathways that contribute to mesopic vision. The outcome of this work can provide a framework for developing new methods for early detection and monitoring of retinal eye diseases that cause blindness, including age-related macular degeneration, diabetic retinopathy, rod-cone dystrophies and retinitis pigmentosa.